1. Field of the Invention
This invention relates to a method and apparatus for transmitting high-frequency signals in an optical communication system. Especially, the invention relates to a method and apparatus for transmitting high-frequency subcarrier signals in an optical communication system that utilizes the optical heterodyne detection technique with a dual-mode local light source and is hard to be affected by phase noise of light sources.
2. Description of the Prior Art
It has been theoretically proven that receiving sensitivity of coherent optical communication systems with strong local light is superior to that of conventional intensity-modulation/direct-detection systems. Therefore, the coherent optical communication systems had been looked to as a next-generation optical communication system. However, optical amplifiers have been developed in recent years in order to improve the receiving sensitivity of existing systems, and this, together with the difficulty of controlling the local light source to reduce phase-noise effect from light sources in coherent optical communication systems, means that coherent optical communication systems are not yet practical.
In conventional optical fiber systems transmitting high-frequency signals, an optical carrier from a light source is modulated, amplified if necessary, and transmitted to a remote site. The optical signal is again amplified if the transmission attenuates the signal power, and then demodulated. FIG. 2 shows an example of such a configuration, which comprises a single-mode light source 101, a high-frequency signal 102, an optical modulator 103, an optical amplifier 104, an optical transmission line 105, another optical amplifier 106, a compensator 107 that compensates for the optical-fiber dispersion effect, a photo-detector 108, and a demodulator 109 that demodulates the high-frequency signal detected by the photo-detector 108.
An optical carrier emitted by the light source 101 is modulated with the optical modulator 103 by the high-frequency signal 102 with payload data. The optical amplifier 104 amplifies the modulated optical signal up to the required power for transmission, and then the amplified optical signal is transmitted along the optical transmission line 105. To compensate for transmission loss and insertion loss due to the optical-fiber dispersion compensator 107, the optical amplifier 106 in front of the optical-fiber dispersion compensator 107 amplifies again the optical signal. To eliminate any effect from optical-fiber dispersion arising in the following photo-detection stage, the optical-fiber dispersion compensator 107 performs compensation on a wavelength-by-wavelength basis. The received optical signal is directly detected by the photo-detector 108, and then the photo-detected signal is demodulated with the demodulator 109 to recover the payload data.
In the conventional system, in order to extend the transmission distance, it has been necessary to use multiple optical amplifiers to increase the received signal power to a level large enough to ensure the desired communication quality. However, light that is spontaneously emitted from an optical amplifier is amplified with the following optical amplifiers, causing accumulation of spontaneously emitted light. This spontaneously emitted light cannot be removed any longer in the case of analog optical communication systems. Once it is detected by the photo-detector, the spontaneously emitted light forms noise that corrupts the desired signal quality. The noise is well-known as an amplified spontaneous emission (ASE) noise. Moreover, in the conventional system additional optical-fiber dispersion compensators must be added to nullify the effect of the optical-fiber dispersion. However, since the dispersion effect strongly depends on both the wavelength and the transmission distance involved, the optical-fiber dispersion compensators must have been individually adjusted, making the configuration of the optical communication systems more complex.
In view of the foregoing, an object of the present invention is to provide an optical communication system not only that does not need to use any optical amplifiers that give rise to ASE noise, but also in which phase noise of light sources is removed in principle.
Another object of the invention is to provide a system that is not affected by optical fiber dispersion effect, thereby eliminating the use of additional optical-fiber dispersion compensators that are required in the conventional system.